1. Field of the Invention
The present invention relates to a liquid crystal display device for performing color display that is used in a color television, a personal computer or the like, and to particularly a liquid crystal display device for providing three primary color display by time-sharing, and providing full color display by mixing the three primary colors without using any color filter.
2. Related Background Art
In recent years, color liquid crystal displays have grown in demand due to advancement of personal computers.
In liquid crystal display devices that are currently on the market, color filters for three primary colors of red (R), green (G) and blue(B) are placed in positions corresponding to pixels, backlights are placed on the back face, and white light is applied to obtain color images.
On the other hand, a color liquid crystal panel of field sequential mode that has a liquid crystal panel of monochrome display and backlights each capable of illuminating lights of three primary colors to perform color display by time-sharing without having any color filters has been proposed.
First, a color liquid crystal display device of field sequential mode using RGB three-color light sources will be described as a conventional example 1.
FIG. 11 is a block diagram showing a configuration of the above-described color liquid crystal display device. In FIG. 11, reference numerals 11 to 13 denote AID (analog/digital) conversion circuits, reference numeral 20 denotes a P/S (parallel/serial) conversion circuit, reference numeral 21 denotes a memory, reference numeral 22 denotes a liquid crystal display part, and reference numeral 23 denotes a light source unit.
In the liquid crystal display device of FIG. 11, signals of three primary colors of R (red), G (green) and B (blue) included in an inputted color image signal are inputted to their input terminals, and digital conversion processing is carried out in the AD conversion circuits 11 to 13. R, G and B digital signals outputted from the A/D conversion circuits 11 to 13 and a synchronous signal Vsync are supplied to the P/S (parallel/serial) conversion circuit 20. The P/S conversion circuit 20 comprises a memory 21, and inputted R, G and B digital signals are serially outputted at a threefold speed from the P/S conversion circuit 20. The threefold-speed digital signals are supplied to the liquid crystal display part, and are subjected to analog conversion in a drive IC (not shown). Also, similarly, synchronous signals Fsync are generated based on the synchronous signal Vsync supplied to the P/S conversion circuit 20, and are synchronously separated from each other and supplied to the liquid display part 22 and the light source unit 23, respectively.
In the liquid crystal display part 22, the supplied threefold-speed digital signals are subjected to analog conversion to display an image, and in the light source unit 23, light source controlling signals of respective colors are generated based on the supplied synchronous signal Fsync, and R, G and B light sources are successively lit based on timing of the light source controlling signals, as shown in FIG. 15.
In FIG. 15, reference characters BLR, BLG and BLB denote timings of lighting of R, G and B light sources, respectively, reference character 1F denotes one frame, reference character if denotes one field, reference character LC denotes the light transmittance (maximum transmittance is 100%) of the pixel in 100% gray level display, and reference character T denotes brightness of light caught by observer's eyes.
Furthermore, in FIG. 15, a state of transient transmission due to delay of speed of response by the liquid display part and delay at the time of on/off of the light sources of three primary colors is not considered.
As shown in FIG. 15, the R light source is lit for the field in which the R image is displayed on the liquid crystal panel 22, the G light source is lit for the field in which the G image is displayed thereon, and the B light source is lit for the field in which the B image is displayed thereon. In this way, by successively displaying the R, G and B images, full color images can be displayed using light persistence in the eye.
In a liquid crystal display device that performs color display in plane sequential mode, no problems arise when a static image is displayed, but, for example, in display of dynamic images in which a white image (image represented with two or more of R, G and B colors) moves on the screen, a “color sequential artifact” (hereinafter abbreviated as “CSA”), in which coloring occurs before and after movement of the dynamic image due to time difference among R, G and B fields, occurs. Also, conversely, the color sequential artifact (CSA) similarly occurs when the line of an observer's sight is shifted. This situation is schematically shown in FIGS. 12A and 12B. In FIGS. 12A and 12B, reference numeral 121 denotes the line of an observer's sight, reference characters n and n+1 denote any sequential frames, reference character ΔX denotes the amount of movement of the dynamic image from the n frame to the n+1 frame, and reference character t denotes time.
FIG. 12A shows the color sequential artifact (CSA) occurring when the observer shifts the line of sight in the left to right direction over the drawing, in the case where a white display (W) image obtained by mixing R, G and B is displayed at the time of the displayed background color of black (B). As shown by the line of sight of FIG. 12A, assuming that the line of sight of the observer making an observation with the G field at the center is shifted, the position on the retina relative to the line 121 indicated by the line of the observer's sight is varied for each of R and B fields. Therefore, the position of light remaining on the retina is varied for each of R, G and B fields, and thus as shown in FIG. 12B, coloring of cyan (C) and B occurs on the left side of the W image, and coloring of yellow (Y) and R occurs on the right side of the image. Also, a similar phenomenon occurs when a person looking at something outside the screen rapidly shifts the line of sight to the screen. Also, such a phenomenon is typically observed when a highly bright and colorless image is moved in a dark background image, even when the line of sight is fixed.
For a method of preventing the color sequential artifact, there is a method in which the field frequency is increased, in the first place. However, for example, if horizontal and vertical scan frequencies are increased by two times compared to the conventional frequencies (the field frequency is increased to a sixfold-speed), for example, power consumption is increased due to enhancement of the speed of data transfer, the speed of response by the liquid crystal is reduced to provide only poor display, and so on, thus causing other problems to arise.
A second method of the conventional technology is a method in which four fields including three fields of primary R, G and B colors and a white field (hereinafter referred to as “W field”) are successively driven in order to alleviate the above problems. FIG. 13 is a block diagram showing the configuration of a device for performing this method. In FIG. 13, reference numeral 14 denotes a minimum value detection circuit, reference numerals 17 to 19 denote subtraction processing circuits, and members identical to those in FIG. 11 are denoted by the same reference characters.
In the device shown in FIG. 13, as in the case of the device of FIG. 11, R, G and B signals included in inputted color image signals are inputted in their individual input terminals, and are subjected digital conversion in A/D conversion circuits 11 to 13. The signals of R, G and B colors and a synchronous signal Vsync outputted from the A/D conversion circuits 11 to 13 are supplied to the minimum value detection circuit 14, the minimum value detection circuit 14 compares the inputted R, G and B digital signals, and supplies the minimum value thereof to the P/S conversion circuit 20 as the W signal. At the same time, the minimum value detection circuit 14 supplies the value to the R, G and B subtraction processing circuits 17 to 19. Also, the minimum value detection circuit 14 supplies R, G and B digital signals to the R, G and B subtraction processing circuits 17 to 19, respectively.
The R, G and B subtraction processing circuits 17 to 19 carry out processing of subtracting the W signal (the minimum value of R, G and B digital signals) displayed in the white field from the inputted R, G and B color signals, and R′, G′, B′ and W color signals subjected to subtraction processing are supplied to the P/S conversion circuit 20, and are stored in the frame memory 21. In addition, the synchronous signal Vsync outputted from the minimum value detection circuit 14 is also supplied to the P/S conversion circuit 20.
The parallel R′, G′, B′ and W color signals inputted in the P/S conversion circuit 20 are serially outputted via the memory 21. In other words, a fourfold-speed digital signal obtained by subjecting the R′/G′/B′/W color signals to time-sharing is supplied to the liquid crystal display part 22 of monochrome display. Also, signals Fsync generated based on the signal Vsync inputted in the P/S conversion circuit 20 are synchronously separated from each other and supplied to the liquid crystal panel 22 and the light source unit 23, respectively.
In the liquid crystal display part 22, the supplied fourfold-speed digital signal is subjected to analog conversion to display a monochrome image. On the other hand, in the light source unit 23, light source controlling signals of respective primary colors are generated based on the supplied synchronous signal Fsync and light sources of R, G, B and W (the white is obtained by simultaneous lighting of R, G and B light sources) are successively lit based on the timing of the light source controlling signals, as shown in FIG. 16. Furthermore, reference characters in FIG. 16 are same as those in FIG. 15.
In the liquid crystal display part 22, the field where the R image is displayed is irradiated with light from the R light source, the field where the G image is displayed is irradiated with light from the G light source, the field where the B image is displayed is irradiated with light from the B light source. In addition, the field where the W image is displayed is irradiated with lights from the R, G and B light sources at the same time to irradiate the liquid crystal display part 22 with white light. In this way, by successively displaying images of R, G, B and W, full color images are displayed using the light remaining property of the retina.
In the meantime, for the liquid crystal panel, the R light source is lit during display of the R image, but a part of the R signal outputted to the liquid crystal panel is used as a white signal, and therefore brightness for the R color is reduced in proportion to the amount of the part used, and the R color becomes less noticeable. The same is applied to G and B, and as a result, the CSA is less noticeable compared to the conventional example 1.
As shown in FIGS. 14A and 14B, by displaying the W image, the color sequential artifact can be curbed even when the line of sight is shifted and when a quick-motion image is displayed.
However, the method of the conventional example 2 including the W field has an increased power consumption of the light source and an inferior efficiency of light usage, in comparison with the display method of the conventional example 1.
In the RGB system, when the white image is displayed by mixing the three primary colors of light sources, a signal having the maximum level of transmittance in each field of R, G and B should be given to the liquid crystal display part, while each of R, G and B light sources should be lit for the time period corresponding to ⅓ of one frame as shown in FIG. 15. As a result, for the white image, the observer observes brightness corresponding to ⅓ of one frame.
Similarly, when the white image is displayed with a RGBW system constituted by four fields of R, G and B fields plus a W field, brightness signals inputted in the liquid crystal display part are all used as display information of the W field, and therefore their transmittance is 0% in each of R, G and B fields and the white image is displayed with the brightness signal having the maximum transmittance only in the W field. On the other hand, for the light source, the R light source is lit twice covering the R field and W field, and similarly other light sources have their lighting time periods increased by two times. Thus, as shown in FIG. 16, brightness corresponding to each of R, G and B light sources being lit for the time period corresponding to ¼ of one frame is observed.
Therefore, if brightness levels of R, G and B light sources in FIGS. 15 and 16 are the same, the brightness for the RGBW system is ¾ of the brightness for the RGB system when the brightness for the RGB system and the brightness for the RGBW system are compared with each other. Also, for the time period over which each light source is lit in each frame, each of R, C and B light sources is lit for the time period corresponding to ⅓ of one frame for the RGB system, while each of the light sources is lit for the time period corresponding to ½ of one frame for the RGBW system, and therefore power consumption of the light source for the RGBW system is 1.5 times larger than that for the RGB system. As a result, efficiency of light usage for the RGBW system is reduced by ½ in comparison with that for the RGB system.
The object of the present invention is to solve the above problems, and restrain the color sequential artifact and reduce power consumption of light sources in a liquid crystal display device providing color display in field sequential mode.